Why food chain cannot have trophic level more than five
As one progress through each trophic level, each organism on that level loses energy gained from the previous trophic level. This is due to losing energy through wasted material (e.g. bones) and to life processes, 7 of which can be remembered by MRS GREN:
This means that each time, energy is wasted and lost from the food chain. More than five trophics would result in so much energy being lost from the initial trophic (producer), that it would be largely pointless.